As optical filters, simple Michelson interferometers and Mach-Zehnder interferometers suffer from a lack of flexibility in their bandwidth characteristics. The bandwidth is a function of the differential optical path dispersion, which is fixed by the optical path, and has limited tunability. It is therefore considered highly desirable to provide for modified Michelson or Mach-Zehnder interferometers that overcome the deficiencies of conventional interferometers, by allowing the wavelength and bandwidth of these devices tunable.